This invention relates to additives for rubber compounds and in particular to organosilane coupling agents for rubber compounds containing siliceous fillers used in tire building.
A rubber compound, such as for use in a tire, typically includes natural and/or synthetic rubber, process aids, reinforcing filler, and a vulcanizing agent, such as sulfur. The process aids are used to decrease viscosity, disperse filler, reduce shrinkage, and/or provide lubrication. The reinforcing filler is used to increase the strength, hardness, and abrasion resistance of the rubber compound. The reinforcing filler is typically comprised of carbon black and/or a siliceous material, typically silica.
Carbon black is typically used as the predominant reinforcing filler in a rubber compound, especially in a tire component, such as a tread. It has been recognized, however, that using a siliceous material as the predominant reinforcing filler in a tire tread provides benefits, including improved rolling resistance and traction.
A siliceous material is hydrophilic and, thus, is not as compatible with rubber as carbon black. Coupling agents, however, have been developed to increase the compatibility between a siliceous material and rubber. Nonetheless, rubber compounds that are highly filled with a siliceous material tend to be viscous and stiff, and therefore difficult to process.
Organosilane compounds are often used as coupling agents. Trialkoxyorganosilane polysulfide compounds have been found to be particularly suitable for use as coupling agents, especially bis(3-triethoxysilylpropyl)-tetrasulfane (hereinafter referred to as "TESPT"). TESPT and other organosilane compounds, however, are liquids at room temperature, which is undesirable. In large scale industrial processes, liquid compounds are difficult to store, weigh, and otherwise handle.
In order to improve handling characteristics, liquid organosilane compounds have been combined with carriers to create coupling agents in powder and granulate forms. An example of such a coupling agent and a method of forming the same is disclosed in U.S. Pat. No. 4,128,438 to Wolff et al., which is incorporated herein by reference. Wolff et al. discloses mixing TESPT with carbon black in order to obtain a granulate coupling agent. Such a granulate, however, does not substantially improve the processing characteristics of rubber compounds that are highly filled with a siliceous material.
Based upon the foregoing, there is a need in the art for an organosilane coupling agent in granulate or powder form that improves the processing characteristics of rubber compounds that are highly filled with a siliceous material. The present invention is directed to such a coupling agent.